Going to 16 latter is always risky.
Memory is sold in kits for a reason.
The memory in that kit are both from the same sheet of silicon and tested and garanteed from the factory to work together.
If your friend gets one 8gb stick now and another latter, even if the exact same model, there is no garantee they will play nice and thus cost more for your friend to have to buy 8 now, and a 16 kit latter.
Now is this a completley new build, or just going to 8/16gb from 4gb in an already purchased system?
Reason I ask is that for a new build those are poor part choices because they are 2 generations old and there are better priced options out there.